Deck footing size calculator
A footing has two dimensions and they answer different questions. Width spreads the load so the soil does not sink — that is this tool. Depth gets below the frost line so the footing does not lift — that is a local number your building department publishes, and no calculator can know it.
Diameter is rounded up to the next even inch. Depth is a separate question and is set by your local frost line — this tool does not guess it.
What it counts
Tributary area is the patch of deck one post holds up: half the joist span, multiplied by the post spacing along the beam. Multiply that by the load per square foot and you have the pounds landing on one footing.
Divide by the soil's presumptive bearing capacity and you get the bearing area the footing needs. Convert that circle to a diameter and round up to something you can actually dig.
Every step is arithmetic. The only judgement is the soil class, and that is not yours to judge — it is assigned by the building department, and it can require a soils investigation on a difficult lot.
Worked example
A 12-foot joist span with posts at 6-foot centres puts a 6 × 6 = 36 square foot patch on each footing. At the residential 40 + 10 psf that is 1,800 lb. On a 2,000 psf soil the bearing area needed is 0.9 square feet — a circle of roughly 13 inches, rounded up to 14.
Widen the post spacing to 8 feet and the same beam puts 48 square feet on each post: the footing grows, and because area goes with the square of the diameter, it grows less than you would guess. That is often the right trade, and the beam tool is where you make it.
Depth is a different number
Nothing on this page tells you how deep to dig. Depth is set by the local frost line and by the requirement that the footing bear on undisturbed soil, and both are local. Get it from your building department's deck handout before you rent an auger — the full argument is in footing depth.
Code requirement
Footing size and bearing are governed by IRC 2021 R507.3 as adopted locally, and holes are normally inspected open before concrete. Your jurisdiction may amend the model code or adopt a different edition. Confirm with your building department before you cut.